Remote sensing of chemical species below, on top and above the ground, or water is becoming increasingly important for security, conservation and monitoring by both government and private industry. Numerous detection techniques including laser based differential absorption lidar (DIAL) have been used. Each technique is subject to real world noise factors. Even in a differential technique, such as DIAL, which is designed to reduce noise factors, factors such as atmospheric interference, irregular surface reflectance, object interference (bushes, trees, power line, etc.), source laser misalignment, source laser energy distribution and relative motion between the remote sensing platform and the target may lead to poor data quality. This poor data quality, in turn, may lead to missed detection or false positive detection.
For most DIAL techniques, conventional systems use two lasers (or one laser that produces two wavelengths) in which one laser is designated the online laser and the other laser is designated the offline laser. The online laser signal is designed to be absorbed by the chemical species of interest, while the offline laser signal is designed not to be absorbed by the chemical species of interest. By measuring the transmitted and returned energies for both lasers and applying a differential data processing technique, one may measure the path-integrated concentration, or concentration path-length product (also referred to as the concentration path length (CPL)) of the chemical species in the column of air to a particular target location. This differential measurement helps reduce noise factors but the measurement may further be refined by allowing a longer sample exposure or by integrating samples of the same column of air. However, this tends to defeat one of the advantages of a remote sensing system which is rapid area coverage.
The present invention provides a system and method for improving the quality of detection of a chemical species using a DIAL-type of remote sensing system. As will be explained, the system and method uses three or more laser sources and produces at least two CPL samples for a given target location. In addition, the system and method of the present invention uses a correlation technique to obtain improved quality control of the detection process and, simultaneously, permits completion of area survey work in a rapid manner, without missing a detection or reporting a false detection.